Microdata is a specification to embed machine-readable data in HTML documents. Microdata consists of name-value pairs (known as items) defined according to a vocabulary. A collection of commonly used markup vocabularies are provided by schema.org.

The basic syntax includes the itemscope attribute to define an item and the itemprop attribute to describe one of the item's properties. Types are specified using the itemtype attribute and can assume values defined in the vocabulary of choice. For instance, schema.org defines types such as http://schema.org/Person or http://schema.org/PostalAddress.

The following example (from Wikipedia) shows how to use microdata to describe a person:

<section itemscope itemtype="http://schema.org/Person">
  Hello, my name is
  <span itemprop="name">John Doe</span>,
  I am a
  <span itemprop="jobTitle">Graduate research assistant</span>
  at the
  <span itemprop="affiliation">University of Dreams</span>
  My friends call me
  <span itemprop="additionalName">Johnny</span>
  You can visit my homepage at
  <a href="http://www.example.com.com" itemprop="url">www.example.com</a>
  <section itemprop="address" itemscope itemtype="http://schema.org/PostalAddress">
    I live at
    <span itemprop="streetAddress">1234 Peach Drive</span>
    <span itemprop="addressLocality">Warner Robins</span>
    <span itemprop="addressRegion">Georgia</span>.

Check the Schema Validator tool to debug your markup and validate it against the supported schema.